fix(R-1250): the line-test hold waits until the held app has really slowed down before asking the agent

Measured on demo-hp 2026-10-27: qBittorrent 5.2.4 kept sending at full speed for ~14 s after its limit fell to
1 KiB/s; the agent reads the uplink 5-15 s after the call, so every test on request was refused "not quiet"
(four tries in an hour, 4.0-4.2 Mbit/s). waitDrained polls each held app's own upload (qBittorrent up_info_speed,
Transmission uploadSpeed) until <= 16 KiB/s twice, 2 s apart, at most 90 s; an unreadable speed waits 20 s.
Tests TestR1250_* (five), red-proved by four mutations.

controller/internal/upwatch/linehold_test.go

@@ -20,6 +20,24 @@ type fakeTorrent struct {
 	writes    []HeldLimit
 	failWrite bool
 	failRead  bool
+	rates     []int64 // UploadRate answers in order, the last one repeating; none = 0 (already quiet)
+	rateReads int
+	failRate  bool
+}
+
+func (f *fakeTorrent) UploadRate(context.Context) (int64, error) {
+	f.rateReads++
+	if f.failRate {
+		return 0, errors.New("transfer/info: HTTP 404")
+	}
+	if len(f.rates) == 0 {
+		return 0, nil
+	}
+	i := f.rateReads - 1
+	if i >= len(f.rates) {
+		i = len(f.rates) - 1
+	}
+	return f.rates[i], nil
 }
 
 func (f *fakeTorrent) Stack() string {